The long-term goal of the proposed research is to elucidate fundamental molecular principles that govern catalysis in enzymes, in the context of metal-radical pair generation and coupled radical-mediated reactions. This goal will be enacted through a comprehensive program of biochemical and physical studies of the coenzyme B12 (adenosylcobalamin) - dependent enzyme, ethanolamine ammonia-lyase, from Salmonella typhimurium. We have developed new methods, software and hardware for pulsed-electron paramagnetic resonance (EPR) spectroscopy. In conjunction with biochemical techniques, we have developed low-temperature systems and time- resolved EPR approaches that enable pioneering experimental efforts that address, directly, covalent bond- breaking/bond-making reactions in ethanolamine ammonia-lyase at single-step resolution. The general aims are to define the free energy landscape that guides the reactions, identify the origins and characterize the roles of the protein and coupled solvent fluctuations that propel the system over this landscape, and resolve the Angstrom-scale molecular structures that frame the free energy landscape. The projects develop a proposed configurational catalysis model for enzyme catalysis, and seek to establish the radical rearrangement as a paradigm for defining landscape and protein-solvent motional contributions to chemical steps in enzyme reactions. Contributions of solvent structure and dynamics to the enzyme reactions are also addressed by a suite of EPR spectroscopic and relaxation techniques. The outcomes of the research will impact molecular approaches in biomedicine and human health, including anti- microbial therapies, rational design of catalysts, inhibitors, and bio-nanocompartments, and control of free radical reactions.